This invention relates to shaped polymeric articles, especially self-supporting films, having improved receptivity to organic coatings and to methods of treating such articles to achieve this effect.
For many years it has been common to employ self-supporting polymeric films as the backing for normally tacky and pressure-sensitive adhesive tapes. Many films, however, do not inherently possess a surface to which pressure-sensitive adhesive bonds firmly enough to provide a satisfactory tape product. Among such films are cellulose acetate, polyester, nylon, and especially polyethylene and polypropylene, which have extremely low energy surfaces. A number of techniques have been employed to increase the receptivity of such films to subsequently applied coatings, involving coextrusion with carboxylated polypropylene, exposure to various types of radiation or, more commonly, application of a primer solution. Polyethylene and polypropylene, for example, are often treated with a chlorinated polymer (e.g., "Hardlen" 13-LP, which is a 5-10% solution of chlorinated polypropylene available from Toyo Kasei Kogyo Co., Ltd., or Eastman CP-343-1 and CP-343-3, which are functionally equivalent solutions in xylene); for convenience, all such primers will be referred to hereinafter as "Hardlen primers". In order to be effective, the Hardlen primer must be applied in an amount sufficient to leave a coating that is at least 10 nanometer thick, thinner coatings failing to provide adequate bonding.
Although the Hardlen primer is effective when applied to polyethylene, polypropylene, or other film surfaces, it requires an additional processing step. Since one of the main reasons for using films such as polyethylene and polypropylene is to reduce cost, the extra priming step tends to offset the advantages of the comparatively inexpensive base film. In addition to the costs incurred in evaporating solvent, curing the chlorinated polymer (or both), there are other expenses in recovering the evaporated solvent and preventing pollution of the atmosphere.
U.S. Pat. No. 4,405,715 describes and claims a process of washing a polyolefin surface in an organic solvent and thereafter plasma treating it in the presence of an "active gas" to improve receptivity to subsequently applied coatings, the exact procedure followed in measuring adherability being difficult to ascertain. The examples all employ oxygen as the active gas, but bromine, ammonia, ozone, and chlorine are also indicated to be acceptable. Each of these gases presents handling problems, and chlorine is sufficiently toxic that it would be avoided in commercial manufacturing procedures.